High Conductivity, Precision Printing Paste Preparation Techniques

Thick film silver paste, which sintering after printed on the glass substrate, then forming high conductivity and fine line pattern is especially important for typical applications include automobile shield defog heating line and plasma panel display (PDP) bus electrode. The challenges of these material are screen printing and sintering process. Many long fine patterned lines should be printed on a large piece of glass and sustain a smooth, continuous and uniform line pattern; on the other hand, the sintering process is rapidly increasing sintering temperature to a peak in a short time, then cooling the glass substrate to low temperature under a short time shock. The total sintering time is limited within 250 seconds, the common silver paste often make the film deformation, smooth rough, crack and needle pore distribute. In order to make these silver paste successfully used in commercial, a clear understanding is required for the complex relation between different parameters such as particles size, solid volume fraction, packing density on suspension printing and deformation and the mechanism of silver powder sintering shrinkage and the inorganic adhesive effect under the batch manufacture condition.The main emphasis of our researches was to improve the silver paste properties by developing a reliable, screen-printing paste suitable for fine lines and lower sintering porosity and good interface between thick film and glass substrate.In this thesis, these researches have been completed, and the results are as follows:1. Different morphology silver powder with spheres, rods, and flake crystal silver particles have been obtained. It has been obtained manufacture process of 10 kilogram nano silver powder industry scale, which the diameters is about 80nm, bulk density is 1.48 g/cm~3 ; the silver rods prepared method and effective control condition also was obtained, which the diameters is about 90nm,the length is more than 20μm. The batch amount are up to 100g.2. The thesis focused on determining the particles parameters necessary for producing successful suspensions. While viscosity of the silver paste determines its rolling behavior, the gravity induced settling of large particles often cause line deformation. Three kinds of sphere silver powders have been taken to prepare silver paste. Screen printing properties were tested, the line surface smoothing and the edge deform ratio have been analysis by the optic microscope and Scan electronic microscope (SEM) graphs, the formula of the rolling liquid film thickness on particle surface has been deduced, it demonstrated that the particles diameters and the solid volume fraction affect the sliding thickness of organic medium and the silver paste rheology. Base on the practical printing condition, we ensure that the liquid film thickness between the particles in the range 0.1～0.9μm, can make the suspension printing film surface roughness change ratio below the 10%, the line edge deforming ratio below the 10%, the formula can be as a predict according to sphere silver particles, the printable solid content under its certain diameters.3. Nano-size silver powders were added to micro-size powders as the effective method to reduce the porosity of the silver film, as the smaller particles can fitted into the interstitial spaces between the larger spheres, it has been displayed that the silver paste rolling behavior has been amend. Meanwhile, it also overcome many limitation in current screen-printing schemes by take the measure of powder surface modified, the printing line pattern film possesses dense, smooth and lower deformation at the silver concentration up to 80%.4. The thesis demonstrate that particles agglomerate and particles contact numbers affect on the silver film microstructure evolution, through analyzing the sintering microstructure and the pore distribution by BET,SEM,DTA, it discusses the internal link of the powder sintering activity and particles agglomerates with the film crack, needle, shrinkage.5. The thesis discussed the glass substrate change under the tough process, associated with the preliminary silver paste possessing porosity film, It present to take use of bimodal free-lead glass enamel frit to control the liquid phase appear and quantities and the silver compact take use of the nanosize particles fill in the void, to decrease sintering neck contact shrinkage, at the last, obtained the silver paste process match with the glass tough treatment, reach the technique target.6. In this thesis, lithographic processes forming electrodes of PDP with photosensitive pastes is investigated , the optimum parameters of the technique was validated with experiments. The feasibility of the rods silver powder and the flake crystal silver powder prepare the photosensitive pastes is also investigated, the results are: silver rods make the silver film is not so smooth, but if it added in sphere nanosize particles and micro-size silver powders at the certain ratio, it also obtained good printing and film density, the flake crystal silver powders are the best conductivity for the silver film.The silver paste material not only match with the car's defog shield heating line, PDP bus electrode, it also suit for those which printing electrode on the glass substrate such as VFD, hybrid multi-layers electrodes and fine line electrode. These material possess widely prospect and potential market.